It is well recognized in the ophthalmic glass art that multifocal lenses can be prepared by sealing one or more small segments or buttons of a glass having a high refractive index into a recess formed in a major lens blank molded from a crown glass. In the customary commercial practice, the major lens blank is fabricated from a spectacle crown glass having a refractive index of 1.523 and the segment or button glass has a higher refractive index, conventionally in the range of 1.58-1.71, the selection thereof being dependent upon the degree of visual correction required in the finished multifocal lens. Generally, the glass manufacturer will supply glasses of four refractive indices which encompass that range, those indices being about 1.58, 1.61, 1.65, and 1.70.
Ophthalmic lenses produced from photochromic glasses, or phototropic glasses as such have been variously termed, have recently been introduced into the marketplace. Such glasses are disclosed in U.S. Pat. No. 3,208,860, the basic patent in the field, as having the capability of darkening when subjected to actinic radiation, commonly radiation in the ultraviolet portion of the spectrum, and thereafter returning to their original optical transmission when removed from the incident radiation. Ophthalmic lenses, index-corrected to 1.523, have been marketed by Corning Glass Works, Corning, N.Y. under such registered trademarks as PHOTOGRAY and PHOTOSUN.
U.S. Pat. No. 3,973,976 observed that, until the time of that disclosure, photochromic ophthalmic lenses had been marketed as "single vision" lenses, i.e., as a single lens of photochromic glass having a refractive index of 1.523. Accordingly, there was a need for glasses with higher refractive indices, but which also exhibited the necessary chemical and physical characteristics for use as ophthalmic lenses and, very importantly, for sealing to the major lens blank. That patent explains that the sealing operation involves fusing the segment to the major lens blank. Consequently, the segment glass must possess a softening point, a strain point, and a coefficient of thermal expansion that closely match those properties of the major lens blank such that the seal resulting from the fusion step will manifest a very low stress level. U.S. Pat. No. 3,973,976 discloses and claims glass compositions of high refractive index which have the properties desired for opthalmic applications and can be fusion sealed to photochromic glasses of the type cited above marketed by Corning Glass Works under the designations Corning 8097 and 8098. Those glasses have softening points in the vicinity of 659.degree.-675.degree. C., strain points of about 469.degree.-473.degree. C., and a coefficient of thermal expansion over the range of 0.degree.-300.degree. C. of about 51.times.10.sup.-7 /.degree. C. Glasses disclosed as being suitable for use as segments for such photochromic lenses consisted essentially, in weight percent on the oxide basis, of 3-7% B.sub.2 O.sub.3, 3-11% Al.sub.2 O.sub.3, 30-60% PbO, and 27-55% SiO.sub.2. Useful optional additions included up to 6% La.sub.2 O.sub.3, up to 10% BaO, up to 3% Li.sub.2 O and/or Na.sub.2 O, and up to 3% TiO.sub.2. The sum of all additions to the base quaternary, however, was limited to no more than 10%. Those glasses were not photochromic in themselves but, because the segment constitutes only a small portion of the composite multifocal lens, it does not detract very substantially from the photochromic behavior furnished by the major lens blank.
Very recently, Corning Glass Works has marketed a new photochromic ophthalmic lens under the mark PHOTOGRAY EXTRA and given the designation Corning 8111. This glass, index-corrected to 1.523, is encompassed within U.S. application Ser. No. 14,981, filed Feb. 28, 1979 in the names of Hares, Morse, Seward, and Smith, and possesses a softening point of 662.degree. C., a strain point of 468.degree. C., and a coefficient of thermal expansion over the range of 0.degree.-300.degree. C. of 64. Because of the substantial difference in thermal expansion existing between the Corning 8111 glass and the Corning 8097 and 8098 glasses, different segment glasses for sealing thereto were obviously demanded to insure fusion seals of very low stress.